Conventionally, an electrical resistance heater used in a semiconductor process or an optical process has been of a type composed of a support base made of a sintered ceramic such as alumina, aluminum nitride, zirconia, boron nitride or the like, to which a wire or a foil of a high melting point metal such as molybdenum and tungsten is attached by winding or via adhesive as the heating element, with an electrically insulating ceramic plate mounted thereon, or of a type which is made by embedding the heating element directly in a support base and sintering them together at once. Further, there has been developed an improved electrical resistance ceramic heater in which a heating element layer made of a conductive ceramic is provided over an electrically insulating ceramic support base and the whole system is covered up with an electrically insulating ceramic cover layer, with the result that the electric non-conductance and corrosion resistivity are improved.
The ceramic support base is usually made of a sintered material obtained by sintering a raw powder after an addition of a sintering additive; however, since a sintering additive is added there are concerns that impurities may cause pollution at the time of heating and the corrosion resistivity is decreased. Also, since the support base is of a sintered material it carries a problem in terms of thermal shock resistance, and especially when the size is relatively large the sintering would proceed less uniformly with a result that the sintered body may carry a tendency to incur cracking or the like, so that such a body cannot be useful in a process where a rapid temperature rising or falling is inevitable.
To remedy this, a one-body type electrical resistance heater made of multi-layered ceramic has been developed, which consists of the support base made of pyrolytic boron nitride (hereinafter referred to simply as “PBN”) formed by means of a thermo-chemical vapor phase vapor deposition method (hereinafter also called “thermo-CVD method”), the heating element made of pyrolytic graphite (hereinafter referred to as “PG”) laid over the surface of the support base by means of thermo-CVD, and the densely laminated protective cover layer, made of the same material as the support base, laid over the heating element to cover up the entire heater, also by means of thermo-CVD method.
This kind of multi-layered ceramic heater is widely used as a high purity heater having characteristics of chemical stability and thermal shock resistance in various fields where a rapid temperature rising or falling is conducted, especially in the continuous process or the like wherein semiconductor wafers or the like are treated in one-by-one manner with the temperature controlled step-wise respectively. Also, as all of the constituting layers of this multi-layered ceramic heater are made by means of thermo-CVD method, there exists no grain boundary in it that is found in a sintered ceramic heater made by sintering powder so that the tissue is dense and does not absorb gas whereby no outgas occurs with a consequence that it has increased its popularity as a heater that does not affect the degree of vacuum in an evacuated process.
Also, normally in this kind of a ceramic heater, for the purpose of conducting electricity to the heating element, a hole must be made through an end portion which functions as a terminal and also a part of the heating element constituting the electric passage must be exposed by removing a part of the electrically insulating protective cover layer, which covers the heating element. The current practice is to use a bolt and a washer at the terminal locations to secure electricity passage. When this procedure of fixing by bolt and washer is adopted to effect electric conductance, the screwing up of the bolt may cause the washer to turn slightly, and this may in turn damage the edge of the insulating ceramic protective cover layer in the vicinity and may cause abnormal heating on account of the consequent poor electric contact, whereby the temperature distribution is ill-affected and, without remedying, the exposed part of the terminal can wear out and begins sparking and in the end the circuit may be snapped at the terminal portion.
In this regard, IP Publication 1 discloses a PBN heating apparatus wherein, in order to prevent the above-mentioned problem, a terminal post, which is formed with a female screw so as to threadably receive a bolt, is fixed at a terminal position of a heating element whereby the heater body and the terminal post are made in one body and this one body is coated with an insulating cover layer. However, even with this kind of PBN heating element, a contact failure occurs between the terminal post and the terminal of the heating element after thermal hysteresis giving rise to an abnormal heating and snapping of the circuit, so that still a connection method that can perfectly prevent the above-mentioned trouble is hoped for.